Page 345 - Vitamin D and Cancer

332 B.W. Hollis

detect 25(OH)D , so it will not be a viable product in countries in which vitamin D
3 2
is used clinically, including the United States.

15.3.4 Direct Physical Detection Methods

Direct detection methodologies for determining circulating 25(OH)D include both
HPLC and LC/MS procedures [16–20]. The HPLC methods separate and quantitate
circulating 25(OH)D and 25(OH)D individually. HPLC followed by UV detection
3
2
is highly repeatable and, in general, most people consider it the gold standard method.
However, these methods are cumbersome and require a relatively large sample as
well as an internal standard. Sample throughout is slow and is not suited to a high
demand clinical laboratory processing up to 10,000 25(OH)D assays per day.
Researchers have recently revitalized LC/MS as a viable method to assess
circulating 25(OH)D [17–20]. As with HPLC, LC/MS quantitates 25(OH)D and
2
25(OH)D separately. When performed properly, LC/MS is a very accurate testing
3
method. However, the equipment is very expensive and its overall sample through-
put when performed properly and ease of operation cannot match that of the auto-
mated instrumentation format. As a methodology, LC/MS can compare favorable
with RIA techniques [18, 19]. One unique problem with LC/MS is its relative
inability to discriminate between 25(OH)D and its inactive isomer 3-epi-25(OH)
3
D . This problem has been especially noticeable in the circulation of newborn
3
infants [17]. Next to the DiaSorin assays, LC/MS is the next most utilized procedure
for the clinical assessment of circulating 25(OH)D.

15.4 Clinical Reporting of Circulating 25(OH)D
Concentrations

As highlighted earlier, all DiaSorin 25(OH)D assays are approved by the FDA for
clinical utility. Thus, the diagnostic 25(OH)D tests sold by DiaSorin and IDS
Diagnostics (Fountain Hills, AZ) are under strict FDA control and monitoring for
assay performance and reliability. In what I consider a distributing trend, many
clinical reference laboratories are replacing these FDA-approved test with “home-
brew” LC/MS methods that are diverse and not under FDA scrutiny. The reasons
for this switch in utilization are the “perceived” advantages of LC/MS technology
being more accurate, precise, specific, cost effective, and providing the separate
determination of 25(OH)D and 25(OH)D . First, with respect to accuracy and
2
3
precision, the DiaSorin and IDS RIA methods perform at least as well as LC/MS
methods according to the Vitamin D External Quality Assessment Scheme
(DEQAS) operated out of London, UK. As far as specificity goes, the DiaSorin
tests appear more specific than LC/MS methodology in that the DiaSorin assays
do not detect the inactive 3-epimer of 25(OH)D [17]. Finally, LC/MS assays are
3